This patent application is based upon and claims the benefit of the earlier filing dates of Japanese Patent Application Nos. 2000-326361 and 2001-213671 filed Oct. 26, 2000 and Jul. 13, 2001, respectively, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
This invention relates to a technique of laser beam machining and a method for manufacturing semiconductor devices using the laser beam machining approach, and more substrates such as semiconductor wafers, glass substrates, or resin substrates, and thin films formed on these substrates.
2. Description of Related Art
Since laser beam machining is capable of delineating fine patterns of an order of a micron (xcexcm) without requiring a lithography process, it has been attracting a great deal of attention as an approach to manufacturing semiconductor devices. In producing semiconductor devices, various types of layers, such as resist films, resin films, insulating films, metal films, etc. are formed and laminated on a wafer. Fine machining is needed not only for forming VIA holes, circuit patterns, and interconnections in the laminated layers, but also for selective removal of the laminated layers along the circumference of the wafer for the purpose of preventing dust from arising during wafer transfer, or revealing the manufacturer serial numbers formed in the wafers.
However, if laser beam machining is carried out in ordinary atmosphere, dust adheres to and accumulates on the processed areas. Adhesion of the dust causes poor exposure, short-circuit, and breakdown, which further causes the manufacture yield to drop.
Moreover, since laser beam machining makes use of ablation (i.e., removal of materials as a result of melting and evaporation), the laser beam that illuminates the substrate or the laminated layers of metals (e.g., aluminum alloy, copper, etc.), insulators (SiO2, Si3N4, etc.), resins, etc. often causes damage to the irradiated regions and the area around them.
FIG. 1 illustrates examples of the damage caused by the conventional technique of laser beam machining. FIG. 1A shows damage to a silicon substrate, FIG. 1B shows damage to a metal layer, FIG. 1C shows damage to a Si3N4 film, and FIG. 1D shows damage to a photoresist.
As illustrated in FIG. 1A, a silicon single crystal wafer 1100 is machined in ordinary atmosphere using the fourth harmonic wave of a Q-switch Nd YAG laser, and the cross-sectional view of the machined area is observed by a transmission electron microscope (TEM). Polycrystalline silicon 1101 and void 1101A are formed around the machined area (or the irradiated area 1100), and many dislocation lines 1102 are observed.
Of these, it is thought that polycrystalline silicon 1101 and void 1101A are produced when melted silicon that has been fused by laser-beam irradiation solidifies. Moreover, since a steep temperature gradient is produced around the irradiated region 1110 by irradiation of the laser beam, a large amount of thermal stress accumulates even in domains in which the silicon single crystal wafer 1100 is not fused, and as a result, dislocation 1102 arises. With the deepening of the depth from the surface of the silicon single crystal wafer 1100, dislocation 1102 is apt to increase. At a depth of 200 xcexcm, dislocation 1102 is observed over a wide area with a radius of about 100 micrometers from the center of the irradiated region 1110.
In addition, swelling 1103 of fused silicon arise from the top face of the silicon wafer 1100 around the irradiated area 1110, and silicon grains 1104 scattered by laser beam machining adhere to the swelling 1103 and around it.
This damage is observed even if the energy density of the laser beam is reduced to about 2.5 J/cm2, which is the lower limit of laser-beam processing. Similar damage is observed even if a KrF excimer laser or its analogues are used to process the silicon substrate in ordinary atmosphere. Although machining lasers with a pulse width of several nanoseconds or greater, such as Q-switch Nd YAG lasers and KrF excimer lasers, are comparatively inexpensive and reliable in operation, the damage accompanying the irradiation of the leaser beam can not be avoided.
It is reported that using a laser beam with a very narrow pulse width of 1 picosecond or less can to some extent prevent fusion and the resultant thermal stress caused in a silicon wafer. Titanium sapphire laser is known as such a narrow-pulse laser with a pulse width of 1 psec or less. However, since titanium sapphire lasers are expensive, they are not suitable for processing semiconductor devices.
Moreover, voids 1101A and dislocations 1102 produced in the silicon single crystal substrate during laser beam machining lower the mechanical strength of the silicon wafer 1100, and induce further damage to the circuit elements or interconnects formed on the silicon wafer 1100. Swelling 1103 and scattered silicon grains 1104 will also induce degradation of the upper layers. These defects result in a reduced yield of semiconductor devices.
FIG. 1B is a cross-sectional view of a laser-processed thin metal film (copper, aluminum alloy, etc.) 1130 formed on the silicon single crystal substrate 1100 via a silicon oxidation film 1120. The thin metal film 1130 was machined in ordinary atmosphere using the fourth harmonic wave of a Q-switch Nd YAG layer. Similarly, FIGS. 1C and 1D illustrate a silicon nitride film 1150 and a photoresist film 1160, respectively, processed by the fourth harmonic wave of the Q-switch Nd YAG laser in ordinary atmosphere.
Swelling 1133 arises around the laser irradiation area 1110 on the thin metal film 1110, as in the silicon single crystal substrate 1100 shown in FIG. 1A. A large number of metal grains 1134 are scattered by the irradiation of the laser beam 1140, and they adhere to the swelling 1133 and its surrounding area. The height of the swelling 1133 is about 2 xcexcm to 5 xcexcm, and the diameter of the metal grain 1134 reaches several micrometers. The swelling 1133 and the metal grains 134 deteriorate the reliability of the upper layers, and cause the yield of semiconductor devices to fall.
If the thin metal film 1130 is a cupper film, it is found by scanning micro-auger (xcexc-AES) analysis that carbon (C) contamination 1135 has occurred around the laser-beam irradiation area 1110. Such carbon contamination is conspicuous at the swelling 1133, and the carbon contents reaches as much as a several tens percentage. Generally, the thin metal film 1130 is patterned into interconnections or electrodes. Carbon contamination 1135 partially increases the resistance of the interconnections and the electrodes, and designed circuit characteristics cannot be obtained. These defects also result in the decreased manufacture yield of semiconductor devices.
Swelling and scattered grains are also observed in the silicon nitride film 1150 and the photoresist film 1160. After the silicon nitride film 1150 is laser-beam machined in ordinary atmosphere, swelling 1153 arises around the laser-beam irradiation area 1110, and a large number of silicon nitride grain 1154 adhere to the swelling 1153. Similarly, if laser beam machining is conducted to the photoresist film 1160 in ordinary atmosphere, swelling 1163 and a large number of photoresist grains 1164 that have adhered to the machined surface are observed.
Since the silicon nitride grains 1154 and the photoresist grains 1164 are small compared with the metal grains 1134, these particles scatter over hundreds of micrometers around the laser-beam irradiation area 1110. The widely spread silicon nitride grains 1154 adversely affect the upper thin films formed on the silicon nitride film 1160. The scattered photoresist grains 1164 induce poor exposure and poor development in the photolithography process. In any cases, the manufacturing yield is reduced.
In one aspect of the invention, a laser beam machining method is provided, which includes (1) supplying a liquid, through which a laser beam can be transmitted, to a target surface of an object to be processed, (2) guiding a laser beam to the target surface through the liquid, and (3) procesing the target surface by the laser beam under the application of ultrasonic vibration.
In another aspect of the invention, a laser beam machining apparatus is provided which includes a laser oscillator, a holder configured to hold an object to be processed, an optical system configured to guide a laser beam emitted from the laser oscillator to a target surface of the object, and a liquid supplier for configured to supply a liquid to the target surface of the object. The holder has an inlet port and an outlet port located so as to substantially align with the target surface of the object.
In still another aspect of the invention, a laser beam machining apparatus includes a laser oscillator, a holder configured to hold an object to be processed, an optical system configured to guide a laser beam emitted from the laser oscillator to a target surface of the object, a liquid supplier configured to supply a liquid to the target surface, and a rotation mechanism configured to rotate the object.
In yet another aspect of the invention, a method for manufacturing a semiconductor device using laser beam machining is provided. This method includes (1) forming a film above a substrate, (2) supplying a liquid, through which a laser beam can be transmitted, to a target surface of the film, and (3) guiding a laser beam to the target surface through the liquid, and patterning the film into a predetermined shape by the laser beam, while applying ultrasonic vibration to the target surface.
In yet another aspect of the invention, a method for manufacturing a semiconductor device includes (1) forming a film above a substrate, (2) supplying a liquid, through which a laser beam can be transmitted, to a target surface of the film, and (3) guiding a laser beam to the target surface through the liquid, and selectively removing the film along a periphery of the object by the laser beam, while rotating the object.
In yet another aspect of the invention, a method for manufacturing a semiconductor device includes (1) forming a film above a substrate, (2) supplying a liquid, through which a laser beam can be transmitted, to a target surface of the film, and (3) guiding a laser beam to the target surface through the liquid and processing the film under the condition of Tixe2x89xa70.3/xcex1i, where xcex1 i is the laser-absorption coefficient of the film, and Ti is the thickness of the film.
In yet another aspect of the invention, a method for manufacturing a semiconductor device includes (1) forming a resist film above a wafer via an antireflection film, (2) supplying a liquid, through which a laser beam to be transmitted to a target surface of the resist film, and (3) guiding a narrow laser beam to the target surface through the liquid, and scanning the laser beam on the target surface to remove a predetermined area of the resist film and the antireflection film.